It has long been a practice in the metallurgy industry to employ induction heating means to galvanneal continuous strip metals, like strip steel, with other metal coatings (such as zinc or zinc-alloy) applied as liquids. The induction heating causes increased bonding into alloy phases between the strip material and the liquid metal coating. Galvannealed metals have known advantages over galvanized metals, such as better welding and painting characteristics and improved corrosion resistance.
One of the most demanding applications for galvannealing metal strip by induction heating is heating a steel strip from about 850 degrees to 1050 degrees Fahrenheit after the strip has been galvanized through a zinc bath. This type of strip is used extensively in automotive body panels.
A transverse flux type of induction coil is commonly used to heat thin metal strip. A coil or plural coils of this type are placed adjacent one or both sides of the strip and the strip is heated as it is conveyed past the coils. This arrangement does not completely surround the strip, and thus allows for easy insertion and removal from the strip without access doors or coil disassembly. However, the current flow induced in the strip is in a loop that lies in the plane formed by the flat sides of the strip. This may result in poor temperature uniformity and overheating of the edges of the strip. A transverse flux coil with good heating uniformity is a complex design and very expensive to produce.
For the above reason, induction heating of metal strip is usually done using a coil that is of solenoidal construction. As used herein, the term solenoidal means that the inductor completely surrounds the strip. This causes induced current in the strip to flow around the cross-section of the strip. However, a problem with such a coil is that it cannot be easily displaced laterally relative to the strip, or the strip relative to the coil.
One technique for facilitating removal of strip from solenoidal induction coils is to provide a door in the end of each coil, which opens to allow the strip to pass through the end of the coil. This technique has disadvantages in that opening the door breaks the high current path. Complex mechanisms are required to open the door, close the door and insure that electrical contact capable of high current is re-established. This results in high initial cost and shorter useful product life, and may cause eventual failure of the system due to catastrophic arcing.
The design of the present invention satisfies the requirements to disengage the induction coil assembly and to move it away from the strip without resorting to the use of current carrying doors or other complex mechanisms in the induction coils. The present invention provides the heating characteristics of the solenoid induction coil combined with an open end allowing easy disengagement from the strip.
Where it is necessary to induction heat workpieces of irregular size and shape, an embodiment of the present invention is adapted to have half-turns of the coil sections shaped to closely conform to the specific shape of the workpiece. The heating apparatus is energized by two power supplies operating at a 180 degree phase relation to provide opposing current flows in the respective coil sections.